Embodiments of the invention relate generally to medical imaging devices and other systems employing superconducting magnet systems and, more particularly, to a system and method for superconducting magnet quench protection.
As is known, a coil wound of superconductive material (a magnet coil) can be made superconducting by placing it in an extremely cold environment, (e.g., −269° C. or 4 K). For example, a coil may be made superconducting by enclosing it in a cryostat or pressure vessel containing a cryogen. The extreme cold enables the coil wires to be operated in a superconducting state. In this state, the resistance of the wires is practically zero. To introduce a current flow through the coils, a power source is initially connected to the coils through a superconducting switch that is temporarily operated in a normally conducting state. In the superconducting state, the current will continue to flow through the coils, thereby maintaining a strong magnetic field. In other words, because superconductive windings offer little to no resistance to electrical current flow at low temperatures, the superconducting state of the magnet is persistent. The electric current that flows through the superconducting magnet is maintained within the magnet and does not decay noticeably with time.
Superconducting magnets have wide applications in the field of magnetic resonance imaging (“MRI”). In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrically shaped cryogen pressure vessel containing a cryogen, such as liquid helium. The cryogen vessel is contained within an evacuated vessel and formed with an imaging bore in the center. The main magnet coils develop a strong magnetic field in the imaging volume of the axial bore that, when combined with controlled gradient magnetic fields and RF pulses, act to generate a signal from a patient that is received and processed to form an image. In existing MRI systems, a main magnetic field of 1.5 or 3 Tesla is routinely used to produce vivid clinical images.
When employing superconducting magnets for producing a strong magnetic field, it is important to have a robust protection system in place for responding to a phenomenon of “quenching” that may occur, in which a localized portion of the magnet increases in temperature and loses superconductivity. This localized increase in temperature can burn or damage the superconducting coils of the magnet. In addition, the rapid decrease in the molecular density due to boil-off within the cryogen vessel resulting from a sharp temperature rise reduces the insulating ability normally provided by the liquid helium, resulting in possible voltage breakdown through the gas that can seriously damage the various coils and associated control circuitry of the elements within the cryogen vessel.
In order for protection systems of superconducting magnets to work effectively, such protection systems should provide early quench detection and effective quench-back. As an example, for a magnet wound with superconducting wires having fiber glass cloth insulation, an initial normal zone is likely to grow relatively slowly, due to the cryogen that may be present between the magnet wire turns. It may take a certain amount of time to grow a detectable quench voltage signal of several volts up to tens of volts. By this time, a local hot spot may have a temperature at ˜100K already. If no action is taken, temperature at the spot may rapidly reach an excessive level that would compromise the magnet coil construction. Therefore, it is crucial that the quench signal be detected in a timely manner and that timely action be taken to quench other parts of the magnet coil system. The action is often termed as quench-back. Typically this is achieved through a distributed heater network attached to predetermined strategic locations of the magnet coils.
FIG. 1 shows a typical prior art superconducting magnet protection system for a superconducting magnet having eight magnet coils L1(±)-L4(±) connected in series. The protection system includes two superconducting switches S1, S2 connected in series as part of a closed loop that is formed to support a desired operating current. As shown in FIG. 1, each magnet coil L1(±)-L4(±) is shunted using a respective protection resistor R1-R4 included in the protection system. In operation of the protection system, if one magnet coil L1(−) starts losing superconductivity (i.e., quenching), a voltage will show up in the magnet coil. However, the coil voltage (which is the quench signal) grows rather slowly and will not reach a very high level due to the shunting resistor R1−, thus making detection of the voltage difficult. To overcome the problem, the small voltage/quench signal is used to normalize the second superconductive switch S2. When the switch S2 normalizes, a voltage of a substantially higher amount is rapidly produced across the switch. Upon detection of this voltage, a heater network (not shown) connected across the switch S2 is triggered, and a coil quench-back function is then activated.
As an example, for the magnet protection system of FIG. 1, it may take a time corresponding to up to 20% of total magnet quench time to have a voltage of several volts in the quenching coil, which triggers the second protection switch S2 to derive a useful quench-back voltage. While this configuration of the magnet protection system thus provides for quench detection, it has two main disadvantages. First, the configuration of the magnet protection system of FIG. 1 results in a fairly large unbalanced electromagnetic quench force that must be supported by the cryostat structure. Second, the configuration of the magnet protection system of FIG. 1 requires the use of a second superconductive switch S2.
Another prior art superconducting magnet protection system is illustrated in FIG. 2. In this design, symmetric magnet coil pairs L1(±)-L4(±) (with respect to the magnet mid-plane) are connected first and then shunted by a respective protection resistor. This connection forces electric current to be the same in the symmetric coil pair, thus eliminating the unbalanced quench force existing in the protection system of FIG. 1. Again, the net quench voltage in a shunted section grows slowly and does not reach a very high level due to the shunting resistors R1-R4, as such, it is necessary to use the second switch S2 to generate a higher voltage to drive the heater network (not shown) for coil quench-back. However, the superconducting magnet protection system of FIG. 2 has two disadvantages. One disadvantage is the complexity of coil leads routing since coil leads from one end of the coil former must be jointed together with those at the opposite end. The second disadvantage is that a second superconducting switch S2 must be employed to enable the quench-back.
Other prior art superconducting magnet protection systems, such as the one disclosed in U.S. Pat. No. 5,333,087 to Takechi et al., employ a plurality of diodes in a magnet protection circuit. In such magnet protection systems, the diodes have a predetermined turn-on voltage. Since the diodes shunt symmetrical coil pairs, the disclosed system provides for dumping most energy through only two coils, which is disadvantageous. Additionally, the use of diodes in the magnet protection system significantly increases the cost of the system, as the diodes must be properly sized to carry nearly full operating current.
It would therefore be desirable to provide a magnet quench protection system capable of providing early quench detection and effective quench-back. It would also be desirable to provide a magnet quench protection system that does not produce appreciable unbalanced quench electromagnetic force and that eliminates the need for a second superconductive switch, thus leading to lower cost protection design with less complexity for manufacturing.